Question

I have started using of generics in Delphi 2010 but I have a problem when compiling this piece of code:

TThreadBase = class( TThread )
...
end;

TThreadBaseList<T: TThreadBase> = class( TObjectList<T> )
...
end;

TDataProviderThread = class( TThreadBase )
...
end;

TDataCore = class( TInterfacedObject, IDataCore )
private
  FProviders: TThreadBaseList<TDataProviderThread>;
...
end;

Then I have some nested procedure:

procedure MakeAllThreadsActive(aThreads: TThreadBaseList<TThreadBase>);
begin
...
end;

And finally I want to call this nested procedure in the code of TDataCore class:

MakeAllThreadsActive(FProviders);

But compiler does not want to compile it and it says ( <> brackets are replaced by () ):

[DCC Error] LSCore.pas(494): E2010 Incompatible types: TThreadBaseList(TThreadBase) and TThreadBaseList(TDataProviderThread)

I do not understand it although TDataProviderThread is descendant of TThreadBase.

I had to fix it by hard typecasting:

MakeAllThreadsActive(TThreadBaseList<TThreadBase>(FProviders));

Does anybody know why the compiler says this error?

Answer 1

TDataProviderThread is a descendant of TThreadBase, but TThreadBaseList<TDataProviderThread> is not a descendant of TThreadBaseList<TThreadBase>. That s not inheritance, it s called covariance, and though it seems intuitively like the same thing, it isn t and it has to be supported separately. At the moment, Delphi doesn t support it, though hopefully it will in a future release.

Here s the basic reason for the covariance problem: If the function you pass it to is expecting a list of TThreadBase objects, and you pass it a list of TDataProviderThread objects, there s nothing to keep it from calling .Add and sticking some other TThreadBase object into the list that s not a TDataProviderThread, and now you ve got all sorts of ugly problems. You need special tricks from the compiler to make sure this can t happen, otherwise you lose your type safety.

EDIT: Here s a possible solution for you: Make MakeAllThreadsActive into a generic method, like this:

procedure MakeAllThreadsActive<T: TThreadBase>(aThreads: TThreadBaseList<T>);

Or you could do what Uwe Raabe suggested. Either one will work.

Answer 2

The type

TList <TBase>

is not the parent type of

TList <TChild>

Generics can t be used that way.

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